Known derivative images:
bear, polarbear: Reference photo by Amareta Kelly (https://www.flickr.com/photos/42409752@N07/6266114045/), licensed CC-BY 2.0
gloves_cloth: Reference SVG from Algot Runeman (runeman.org), via openclipart.org; distributed under CC-0.
ape: Original is from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
wolf, dwolf: Derived from base wolf & dwolf, which are derived from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
bone_drake: Original is from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
elf_lass: Derived from clsc elf_lass, which is in turn derived from RLtiles, a CC-0 collection of pixel art. Modified here under CC-0.
messageboard, messageboard_red: Referenced base image messageboard/messageboard_red, licensed under CC-0.
dragonclaw_dagger: Derived from base image dragonclaw_dagger, licensed under CC-0.
file: Derived from base image file, licensed under CC-0.
kite_shield: Derived from base image kite_Shield, under CC-0.
coif: Created by SilverNexus, referencing base image coif. Licensed as CC-0.
lantern, lantern2, lantern_off, lantern2_off: Derived from art by David Gervais. Licensed CC-BY-3.0.
yellow_[0-9], yellow_[A-F], yellow_ne, yellow_nw, yellow_se, yellow_sw: Derived from art by David Gervais. Licensed CC-BY-3.0.
bones3: Redrawn from a David Gervais tile. Licensed CC-BY-3.0.
charcoal: Derived from Saiapatsu's base set charcoal image, licensed CC-0.
misc/go/point, stone_(white|black): Derived from art by Kevin Zheng, licensed CC-BY-3.0.
handmirror: Referenced base set handmirror, licensed CC-BY-4.0.
grand_piano_black, grand_piano_white: Reference photo by Joe Haupt (https://www.flickr.com/photos/51764518@N02/49286256543), licensed CC-SA-2.0
knowledge_generic: Rederived from source of base image (http://commons.wikimedia.org/w/index.php?title=File:HREpolitics.svg&oldid=58248760), licensed CC-BY-SA-3.0.
knowledge_messages: Rederived from source of base image (http://commons.wikimedia.org/w/index.php?title=File:Temple_Scroll.png&oldid=65512408), public domain.
knowledge_monsters: Rederived from source of base image (http://commons.wikimedia.org/w/index.php?title=File:Dragon.svg&oldid=65944895), licensed CC-BY-SA-3.0.
knowledge_recipes: Rederived from source of base image (http://www.nasa.gov/mission_pages/hubble/science/hst_img_20080520.html), public domain.
demonhorn: Reference photo by FarborAnna (https://commons.wikimedia.org/wiki/File:Afrikansk_dv%C3%A4rgget_Kallh%C3%A4lls_4H-g%C3%A5rd_4.jpg)
quest_scorn_kelthior: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:Knowledge-Reid-Highsmith.png), public domain.
quest_scorn_hero: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:Feuille_aldine_du_XVIe_si%C3%A8cle.svg), public domain.
quest_scorn_gates: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:Password.svg) by David Vignoni, licensed LGPL 2.1.
quest_scorn_aristocracy: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:George_Cross_icon.svg) by Zscout370, licensed CC-BY-SA-3.0.
quest_generic: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:Old_map_of_Tianjin_City.jpg), public domain.
quest_darcap_temple_justice: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:3enighed.svg) by AnonMoos, public domain.
quest_darcap_elemental: Rederived from source of base image (http://commons.wikimedia.org/wiki/File:Zywioly03.png) by Ratomir Wilkowski, licensed CC-BY-SA-3.0.

Derivatives of unknown licensing:
capitalhold_west: Referenced from base image capitalhold_west, of unknown licensing.
out_tower: Referenced from base image out_tower, of unknown licensing.
pagoda_tower_east: Referenced from base image pagoda_tower_east, of unknown licensing.
hovels: Referenced from base image hovels, of unknown licensing.
l_shop1: Referenced from base image l_shop1, of unknown licensing.
rl_house3, rl_house_west: Referenced from base image rl_house3, of unknown licensing.
university: Referenced from base image university, of unknown licensing.
treefort, treefort2, treefort3, treehouse: Referenced respective base set images, and derived from retro tree_3, all of unknown licensing.
archblack, archgold, archgreen, archtree, archwood: Referenced respective base set images, all of unknown licensing.
brazier, unlit_brazier, firepot: Referenced base image brazier, of unknown licensing.
lampost: Referenced base image lampost, of unknown licensing.
broken_bird_gen: Derived from retro bird_gen, of unknown licensing.
broken_scorpi_gen: Derived from scorpi_gen, of unknown licensing.
horse: Derived from base image horse, of unknown licensing.
broken_behold_gen: Derived from retro behold_gen, of unknown licensing.
broken_devil_gen: Derived from retro devil_gen, of unknown licensing.
broken_dragoncave, broken_dragoncave_blue, broken_dragoncave_green: Derived from retro dragoncave, of unknown licensing.
ice_warrior: Derived from retro angel_knight, of unknown licensing.
broken_giant_gen: Derived from retro giant_gen, of unknown licensing.
broken_gnoll_gen: Derived from retro gnoll_gen, of unknown licensing.
clown: Referenced from base clown, of unknown licensing.
evilclown: Referenced from base evilclown, of unknown licensing.
nun: Referenced from base nun, of unknown licensing.
broken_bee_gen: Derived from retro bee_gen, of unknown licensing.
chest_1_mimic: Derived from retro chest_1, of unknown licensing.
broken_pixie_gen: Derived from retro pixie_gen, of unknown licensing.
fire_troll: Derived from retro troll, of unknown licensing.
broken_sarcophagus: Derived from retro vampiregen, of unknown licensing.
broken_skull: Derived from skull_gen, of unknown licensing.
bag_empty: Derived from retro bag_popcorn, of unknown licensing.
vial_empty: Derived from retro vial_yellow (or any other), of unknown licensing.
wbottle_empty: Derived from retro water, of unknown licensing.
potion_empty: Derived from the retro potion styling, all of unknown licensing.
crossroad: Referenced from base crossroad, of unknown licensing.
monument: Referenced from base monument, of unknown licensing.
store_food, store_food_barn: Derived from *really* old monochrome food shop image, of unknown licensing.
woodwindow_0, woodwindow_1: Derived from retro woodwall and the style of retro wwindow, both of unknown licensing.
awindow_0, awindow_1: Derived from retro woodwall and the style of retro wwindow, both of unknown licensing.
redcwindow_0, redcwindow_1, westcwindow_0, westcwindow_1, dwindow_0, dwindow_1, dwindow_2, dwindow_3: Derived from retro cwall and the style of retro wwindow, both of unknown licensing.
flagstone_win1: Derived from retro flagstone and the style of retro wwindow, both of unknown licensing.
stwindow_0, stwindow_1, stwindowblue_0, stwindowblue_1, stwindowmossy_0, stwindowmossy_1, stwindowred_0, stwindowred_1: Derived from retro stwall and the style of retro wwindow, both of unknown licensing.
window_0, window_1: Derived from retro wall and the style of retro wwindow, both of unknown licensing.
b_axe1: Derived from base image b_axe1, of unknown licensing.
dhaxe_1: Derived from base image dhaxe_1, of unknown licensing.
dhaxe_2: Derived from base image dhaxe_2, of unknown licensing.
shaxe_1: Derived from base image shaxe_1, of unknown licensing.
handcrossbow, handcrossbow_bone: Derived from base image handcrossbow, of unknown licensing.
reflexbow, reflexbow_bone: Derived from base set reflexbow/reflexbow_bone, both of unknown licensing.
sflail_1: Derived from base image sflail_1, of unknown licensing.
kama_1, kama_2, scythe_1, t_star1, twinblade_spear: Derived from respective base images, all of unknown licensing.
giants_knife, hooksword, rapier, butterflysword_1, butterflysword_2, vlongsword, sicklesword: Derived from respective base images, all of unknown licensing.
altarixalovh, altarsori, altarvalk, mbaltar_none: Derived from David Sundqvist's altar graphics, of unknown licensing.
ipond: Derived from retro grasspond, of unknown origin.
rain1, rain2, rain3, rain4, rain5: Derived from base image rain puddles, of unknown licensing.
broken_gravestone: Derived from retro gravestone, of unknown licensing.
ranger: Derived from retro elf, of unknown licensing.
footpath_[0-F]: Derived from base image footpath_[0-F], of unknown licensing.
bloodreaver: Derived from base image bloodreaver, of unknown licensing.
mbworkbench, mbtanbench: Referenced from their non-mapbuilding counterparts; see tanbench, workbench
mbfirepot, mbbrazier: Referenced from non-mapbuilding counterparts; see firepot, brazier
mb_burningfountain, mbfountain: Referenced from non-mapbuilding counterparts. Burning fountain is derived from fountain. Fountain is of unknown licensing.
tanbench, workbench: Derived from table_4, of unknown licensing.
rl_house2: Referenced from classic image rl_house2, of unknown licensing.
ench_armour, improve_*, prepare_weapon: Derived from retro spell scroll, of unknown licensing.
mb_lavawell: Referenced from retro lava well, which is derived from retro well. License unknown.
mbgreenmarble, mblightmagentamarble, mbmarble, mbpinkmarble: Derived from their respective marble images, all of which are derived from the white marble image. License unknown.
mbwoodfloor, mbwoodfloor2: Derived from their respective woodfloor images. License unknown.
mbblackmarble: Derived from retro black marble image. License unknown.
swall_1_half, swall_2_half, swall_4_half, swall_8_half: Derived from respective retro swall images. License unknown.
quiver_arrow, quiver_bolt: Derived from retro quiver image. License unknown.
water_diamond, water_emerald, water_pearl, water_ruby, water_sapphire, water_wise: Derived from retro water image. License unknown.
amberium_converter, gold_converter, gold_converter2, jade_converter, jade_converter2, platinum_converter, platinum_converter2, silver_converter: Derived from retro converter image. License unknown.

Original images with known licenses:
jade_coin: Created by SilverNexus. Licensed as CC-0.
amber_coin: Created by SilverNexus. Licensed as CC-0.
dresser, dresser2, dresser2_cw, dresser_cw, close_dresser, close_dresser2, close_dresser2_cw, close_dresser_cw: Created by SilverNexus. Licensed as CC-0.
punch_dagger: Created by SilverNexus. Licensed as CC-0.
rain, snow_c: Created by SilverNexus. Licensed as CC-0.
pl_dragon_*, dragon_stoned: Created by SilverNexus. Licensed as CC-0.
knife, fork, spoon, utensils: Created by SilverNexus, only alteration from base version is canvas size. Licensed as CC-0.
dice: Created by SilverNexus. Uses new stylized art that loosely follows in-game depictions. Licensed as CC-BY-SA-3.0.
randomConverter: Created by SilverNexus. Loosely based off the retro converter table art. License CC-0.

Images with attribution of author, but no known license:
altardevo, altargaea, altargnarg, altarvalr, altargoro, altarlyth, altarmostr, altarnone, altarrugg: David Sundqvist (added 0.93.3)

Images that originally shipped with 0.10 (and thus are attributable to Frank Tore Johansen):
armour/helmet/helmet: Derived from 0.10 image, but not identical. Colorized.
- Derivatives of helmet:
-- b_helmet
armour/mail/ring_mail: Colorized, but otherwise appears to be identical.
armour/shield/highshield: Colorized, maybe adjusted? Looks more crowded colorized than not.
door/key1: Colorized, but otherwise appears to be identical.
door/odoor_1: Colorized, but otherwise appears to be identical.
- Derivatives of odoor_1:
-- door/odoor_1_fant_blue-green
-- door/odoor_1_fant_blue-yellow
-- door/odoor_1_fant_brown
-- door/odoor_1_fant_red-white
-- door/odoor_1_fant_red-yellow
door/odoor_2: Colorized, but otherwise appears to be identical.
- Derivatives of odoor_2:
-- door/odoor_2_fant_blue-green
-- door/odoor_2_fant_blue-yellow
-- door/odoor_2_fant_brown
-- door/odoor_2_fant_red-white
-- door/odoor_2_fant_red-yellow
misc/gravestone: Colorized, though barely noticeably. Some shading appears to have been added in colorization.
- Derivatives of gravestone:
-- misc/gravestone2
-- misc/broken_gravestone
monster/animal/mouse: Looks possibly redrawn in addition to colorized.
monster/animal/mouse_gen: Colorized, but otherwise appears to be identical.
spell/Bullet/bullet.112: Appears to be unused in later versions, but was left in the files with a simpler colorization.
spell/Bullet/Fire/fireball: Looks to be redrawn in addition to colorized, but is clearly derived from the 0.10 work.
- Derivatives of fireball:
-- spell/Bolt/firebolt
spell/Bullet/Fire/firebullet: Only N,E,S,W directions are original to 0.10. Diagonals must have come later.
- Derivatives of firebullet:
-- misc/blackfirebullet
-- spell/Bullet/snowball
spell/MagicMissile/magic_miss: Colorized, but otherwise appears to be identical.
wall/wall/wall_*: Except wall_0, wall_55, and wall_AA, the rest are either colorized versions of 0.10 images or derived from them. wall/wall/window walls are also derived from the affected walls.
weapon/axe/axe_1: Colorized, but otherwise appears to be identical.
weapon/bow/arrow: All 8 directional facings are colorized versions of images in 0.10. The stationary face is not.
- Derivatives of arrow:
-- weapon/bow/arrow_assassinating_dragon
-- weapon/bow/arrow_fire
-- weapon/bow/arrow_frost
-- weapon/bow/arrow_lightning
-- weapon/bow/arrow_magic
-- weapon/bow/arrow_paralysis
-- weapon/bow/arrow_poison
-- weapon/bow/arrow_slay_dragon
weapon/bow/bow: Colorized, but otherwise appears to be identical.
- Derivatives of bow:
-- weapon/bow/compositebow
weapon/club/club: Colorized, but otherwise appears to be identical.
weapon/mace/mace_1: Colorized, but otherwise appears to be identical.
weapon/sword/sword_1: Colorized, but otherwise appears to be identical.

Known problem images:
g_worm, purple_worm: These are ripoffs of Final Fantasy worms, and should be replaced.
GreatDemon, medium_demon, fiend: These are ripoffs of the demon designs from 1994 DOS game Dark Legions. GreatDemon is basically a Dark Legions demon posterized to 3 colors, and medium_demon and fiend were derived from GreatDemon.
giantrat: Appears to be derived from an animated rat gif of questionable licensing.

Needs more research:
burnout, fire: Found at https://web.archive.org/web/19981201235415/http://www.ameritech.net:80/users/schmitz/TROOP33.html under the name radioact3.gif, dating back to December 1998. Does not appear to be origin of it, merely a use. More investigation needed to find origin.

Solved problem images:
housesmall: v0.92.0 version appears to be derived from the home icon here: https://www2.ccs.neu.edu/research/demeter/course/projects/demdraw/www/index.html. No known license on the image. However, we can just use the v0.93.2 version and avoid the entire thing.